U.S. Pat. No. 3,023,300 A discloses a process for producing longitudinally welded metal tubes in which a metal strip pulled from a supply reel is shaped into a slit tube in a continuous process using a shaping apparatus that consists of a plurality of shaping roller pairs arranged one behind the other. The strip edges of the slit tube are welded together, the welded metal tube is provided with a helical or annular corrugation, and the corrugated tube is wound onto a supply drum.
Such a process, which is particularly suited for sheathing the cores of electrical cables, makes it possible to produce metal tubes in great lengths.
DE 44 34 134 A1 describes a similar process for producing metal tubes with an outside diameter of between 1 and 6 mm. In this process, too, a metal strip that is continuously pulled from a supply is shaped into a slit tube and the strip edges are welded together. The shaping apparatus also consists of a plurality of shaping roller sets.
A pull-off device with clamping jaws firmly gripping the slit tube precisely guides the slit tube underneath the welding apparatus. Behind the clamping jaw pull-off device, the slit tube runs into a guide unit, which precisely guides the tube in front of and behind the welding point. An opening in the guide unit unblocks the strip edges in a short area, so that a laser beam can weld the edges together.
This process is particularly suitable for producing small tubes made of stainless steel with dimensions of about 3-3.5 mm and a wall thickness of 0.2 mm.
The drawback is that the shaping of the metal strip into a slit tube is limited to a wall thickness of at least 0.15 mm. If the walls are thinner, a precise mutual alignment of the strip edges is no longer possible. However, to obtain a proper weld seam in extremely thin wall thicknesses, it is essential that the strip edges meet exactly. This is true, in particular, if the strip edges are welded by means of a laser welding device.
Thus, the object of the present invention is to improve the initially described process to make it possible to produce tubes with an outside diameter of less than 6 mm from a metal strip with a wall thickness of less than 0.15 mm, particularly less than 0.10 mm.
The features according to the invention offer a surprisingly simple method to produce a flawless metal tube with a very thin wall thickness without requiring any subsequent drawing processes.
It is assumed that the increased friction between the drawing dies and the bent metal strip prevents xe2x80x9cmigrationxe2x80x9d of the bent metal strip or the slit tube. Particularly if a diamond material is used for the drawing dies the drawing dies are not likely to wear, so that large lengths of metal tubing can be produced in a continuous production process.